I. Field of the Invention
This invention relates to a method and apparatus for the purification of water and other aqueous liquids. More particularly, the invention relates to the purification of raw water from a reservoir, river, lake or the like to make it potable and suitable for distribution to consumers, and to the purification of other contaminated aqueous liquids.
II. Discussion of the Prior Art
Raw water from a source such as a reservoir, river or lake must be treated to remove pathogenic bacteria, certain pollutants, solids and colouring materials before it can be distributed to consumers. During the past eighty years or so there have been very few innovations in municipal raw water treatment plants and most comprise the following:
(1) bar screens; PA0 (2) coagulation, followed by flocculation and sedimentation; PA0 (3) gravity feed sand and/or anthracite coal filter beds; PA0 (4) disinfection (super chlorination) and perhaps fluoridation; and PA0 (5) waterworks distribution.
One of the disadvantages of this type of water treatment plant is that the filtration efficiency is not constant. The sand and/or anthracite coal filter beds require the formation of a microbiological sludge layer to be fully established on the surface of the beds before they operate efficiently. Eventually the filters become clogged and have to be fluidized and backwashed, which destroys the sludge layer. Until the layer is restored and completely covers the top of the filter bed, which may take three to twenty four hours, particles of up to 75 microns (0.003 inch) in size may pass through the bed, which means that pathogenic bacteria (e.g. fecal coliforms and pseudomonas) may pass uninhibited through the bed. Once the sludge layer is restored, the bed is capable of trapping particles 1 micron or greater in size. The decreased efficiency of the filter bed requires an increase in the use of chlorine to ensure proper disinfection, but this results in increased corrosion of the distribution system, a bad odour and taste in the drinking water, an increase in the quantities of chlorinated organic and other compounds in the water that may be harmful to health, and increased cost.
Furthermore, sand and coal beds tend to be extremely large, resulting in increases in total plant costs, and are expensive to backwash because large amounts of clean water must be used (and disposed of in a non-polluting way) and steps must be taken to ensure that the beds re-pack properly so as not to leave large channels through which pollutants could easily pass.
Other types of filters have been tried in the past, but those that do not permit the immediate formation of a microbiological sludge layer are not as effective. The activated aerobic sludge layer contains predominantly aerobic, generally non-pathogenic bacteria, protozoa and microorganisms that may trap and/or consume and cause the death of certain pathogenic bacteria and viruses, or at least propagate at the expense of such pathogenic microorganisms by consuming available nutrients. The activated sludge layer also traps solids, including colloidal particles, certain heavy metals and substances that may be precipitated, and reduces the concentration of certain organic chemicals normally present in the water. Thus, the presence of such a layer is extremely effective in the purification of water.